The present invention relates to a technique for measuring the quantity of noise contained in a video signal.
FIG. 2 is a configuration diagram of a conventional noise quantity measuring apparatus described in a patent document 1 (Japanese Unexamined Patent Publication No. 2005-229166).
The noise quantity measuring apparatus extracts, for example, an area A of 10×10 pixels from a first video signal inputted from an input terminal 1 and extracts, for example, an area B of 5×5 pixels corresponding to a set of adjacent pixels with an arbitrary pixel lying in the area A as the center, as a basic image. On the other hand, the first video signal is supplied even to an input terminal 2. The first video signal applied to the input terminal 2 is stored in a storing or memory means 1 and read as a second video signal. Thus, the second video signal assumes a video signal lying immediately before at least one frame from the first video signal.
A computing means calculates differences developed every pixel located in the same position as the area B lying in the extracted basic image and the second video signal. An integrating means 1 determines the sum of the differences and stores the same in a storing means 2a as a correlation value.
Next, the area B is moved by one pixel or plural pixels from side to side and up and down. Similarly, correlation values at their positions are determined and sequentially stored in their corresponding memory or storing means 2b through 2x. A comparing means determines the minimum one of the correlation values stored in the storing means 2a through 2x and supplies the same to an integrating means 2 as the minimum correlation value.
Likewise, the minimum correlation values are determined with respect to all the pixels lying in the area A. The integrating means 2 determines the sum Vn of the minimum correlation values calculated with respect to the respective pixels lying in the area A. The integrating means 2 performs statistical processing of a histogram, a central or medium value, an average value and the like using the value Vn thereby to measure the quantity of noise. Thus, mis-detecting of moving video and a sudden change in image due to a scene change or the like as noise is assumed to be suppressed.
In the noise quantity measuring apparatus, a video signal is divided at a given area, and the minimum value of the differences developed between the video signal and the video signal lying immediately before one frame is determined. Thus, motion based on the differences is suppressed from being mis-detected as noise at the moving video and the image suddenly changed due to the scene change or the like. However, a problem arises in that the noise quantity measuring apparatus cannot adapt to input video of a video camera or the like having an automatic gain control function.
FIG. 3 is a diagram for describing the problems of the conventional noise quantity measuring apparatus.
Assuming that when low-luminance video portions low and high-luminance video portions high are existent in mixed form and noise exists over the whole video as shown in a video signal of FIG. 3(a), for example, a gain control function is operated to bring gain to twice, the luminance level of the high-luminance video portion high exceeds an outputtable range as indicated by an amplified video signal shown in FIG. 3(b). Therefore, the video signal of the high-luminance portion is cut together with a noise component, so that an output video signal assumes a constant value (maximum value) as shown in FIG. 3(c). Thus, when the difference from the video signal lying immediately before one frame is made to determine the minimum value, the difference results in 0 as indicated by a decision signal of FIG. 3(d) because no noise exists in cut portion, so that the noise quantity of each low-luminance video portion low cannot be measured.